1In shelf and coastal ecosystems, planktonic and benthic trophic pathways differ in their carbon stable isotope ratios (δ13C values) and nitrogen stable isotope ratios (δ15N values) and they increase predictably with trophic level. Stable isotope data are therefore used as a tool to study food webs in shelf and coastal ecosystems, and to assess the diets and foraging behaviour of predators. However, spatial differences and temporal changes in prevailing environmental conditions and prey abundance may lead to considerable heterogeneity in stable isotope values measured in focal animal species.